SECURITIES PURCHASE
AGREEMENT(the “
Agreement ”), dated as of October 30, 2006 by and
among Antigenics Inc., a Delaware corporation (the “
Company ”), and the investors listed on the Schedule
of Buyers attached hereto (individually, a “ Buyer
” and collectively, the “ Buyers
”).

A. The Company and each Buyer is
executing and delivering this Agreement in reliance upon the
exemption from registration afforded by Section 4(2) of the
Securities Act of 1933, as amended (the “ 1933 Act
”), and Rule 506 of Regulation D (“ Regulation D
”) as promulgated by the United States Securities and
Exchange Commission (the “ SEC ”) under the 1933
Act.

B. The Company has authorized the
issuance and sale of (i) $25,000,000 in principal amount of
senior secured convertible notes of the Company in the form
attached hereto as Exhibit A (together with any senior
secured convertible notes issued in replacement or exchange thereof
in accordance with the terms thereof and any senior secured
convertible notes issued to pay interest in the form attached
hereto as Exhibit B (“ PIK Notes ”), the
“ Notes ”), which Notes shall be, in accordance
with the terms of the Notes, convertible into shares (as converted,
the “ Conversion Shares ”) of the
Company’s common stock, par value $.01 per share (“
Company Common Stock ”) or into Antigenics MA Shares
(as defined below).

C. The Notes bear interest, which at
the option of the Company, subject to certain conditions, may be
paid in kind through the issuance of additional Notes.

D. Each Buyer wishes to purchase,
severally but not jointly, and the Company wishes to sell, upon the
terms and conditions stated in this Agreement, that aggregate
principal amount of Notes set forth opposite such Buyer’s
name in the Schedule of Buyers (which aggregate principal amount
for all Buyers shall be $25,000,000).

E. The Company is agreeing to pledge
all its shares (“ Antigenics MA Shares ”) in
Antigenics Inc., a Massachusetts corporation (“ Antigenics
MA ”), to secure its obligations to pay interest on and
principal of the Notes and, as evidenced by the Pledge and Security
Agreement, dated as of the date hereof, by and among the company
and the Buyers, substantially in the form attached thereto as
Exhibit C (the “ Pledge and Security Agreement
”) and in connection therewith the Company shall deliver the
number of Antigenics MA Shares.

F. The Notes and the Conversion
Shares collectively are referred to herein as the “
Securities. ”

1. PURCHASE AND SALE OF NOTES
.

(a) Amount . Subject to the
satisfaction (or waiver) of the conditions set forth in Sections 6
and 7 below, the Company shall issue and sell to each Buyer, and
each Buyer severally, but not jointly, shall purchase from the
Company on the Closing Date (as defined below), the principal
amount of Notes set forth opposite such Buyer’s name in the
Schedule of Buyers.

(b) Closing . The closing
(the “ Closing ”) of the purchase of the Notes
by the Buyers shall occur at the offices of Ropes & Gray,
LLP, One International Place, Boston, Massachusetts 02110. The date
and time of the Closing (the “ Closing Date ”)
shall be 9:00 a.m., Boston City Time, on October 30, 2006,
subject to notification of satisfaction (or waiver) of the
conditions to the Closing set forth in Sections 6 and 7 below (or
such later date as is mutually agreed to by the Company and each
Buyer).

(c) Purchase Price . The
purchase price for each Buyer (the “ Purchase Price
”) of the Notes at the Closing shall be equal to the
principal amount set forth opposite such Buyer’s name on the
Schedule of Buyers.

(d) Form of Payment . On the
Closing Date, (i) each Buyer shall pay its aggregate
applicable Purchase Price to the Company for the Notes to be issued
and sold to such Buyer at the Closing, by wire transfer of
immediately available funds in accordance with the Company’s
written wire instructions, and (ii) the Company shall deliver
to each Buyer the Notes which such Buyer is then purchasing, duly
executed on behalf of the Company and registered in the name of
such Buyer or its designee.

2. BUYER’S REPRESENTATIONS
AND WARRANTIES .

Each Buyer represents and warrants
with respect to only itself that:

(a) No Public Sale or
Distribution . Such Buyer is acquiring the Notes, and, upon
conversion of the Notes, will acquire the Conversion Shares, for
its own account and not with a view towards, or for resale in
connection with, the public sale or distribution thereof, except
pursuant to sales registered or exempted under the 1933 Act;
provided, however, that by making the representations herein, such
Buyer does not agree to hold any of the Securities for any minimum
or other specific term and reserves the right to dispose of the
Securities at any time in accordance with or pursuant to a
registration statement or an exemption under the 1933 Act. Such
Buyer is acquiring the Securities hereunder in the ordinary course
of its business. Such Buyer does not presently have any agreement
or understanding, directly or indirectly, with any Person to
distribute any of the Securities.

(b) Accredited Investor
Status . Such Buyer is an “accredited investor” as
that term is defined in Rule 501(a) of Regulation D.

(c) Reliance on Exemptions .
Such Buyer understands that the Securities are being offered and
sold to it in reliance on specific exemptions from the registration
requirements of United States federal and state securities laws and
that the Company is relying in part upon the truth and accuracy of,
and such Buyer’s compliance with, the representations,
warranties, agreements, acknowledgments and understandings of such
Buyer set forth herein in order to determine the availability of
such exemptions and the eligibility of such Buyer to acquire the
Securities.

2

(d) Information . Such Buyer
and its advisors, if any, have been furnished with all materials
relating to the business, finances and operations of the Company
and materials relating to the offer and sale of the Securities
which have been requested by such Buyer. Such Buyer and its
advisors, if any, have been afforded the opportunity to ask
questions of the Company.

(e) Experience of Such Buyer
. Such Buyer understands that its investment in the Securities
involves a high degree of risk. Such Buyer, either alone or
together with its representatives, has such knowledge,
sophistication and experience in business and financial matters so
as to be capable of evaluating the merits and risks of the
prospective investment in the Securities, and has so evaluated the
merits and risks of such investment. Such Buyer is able to bear the
economic risk of an investment in the Securities and is able to
afford a complete loss of such investment.

(f) No Governmental Review .
Such Buyer understands that no United States federal or state
agency or any other government or governmental agency has passed on
or made any recommendation or endorsement of the Securities or the
fairness or suitability of the investment in the Securities nor
have such authorities passed upon or endorsed the merits of the
offering of the Securities.

(g) Transfer or Resale . Such
Buyer will not sell, offer to sell, assign, pledge, hypothecate or
otherwise transfer any of the Securities unless (i) pursuant
to an effective registration statement under the 1933 Act,
(ii) such Buyer provides the Company with reasonable
assurances and customary representations, that such Securities can
be sold pursuant to Rule 144 under the 1933 Act. Notwithstanding
anything to the contrary contained in the Agreement, such Buyer may
transfer (without restriction) the Securities to its affiliates
provided that such affiliate is an “accredited
investor” under Regulation D and such affiliate agrees to be
bound by the terms and conditions of the Agreement.

(h) Legends . Such Buyer
understands that the certificates or other instruments representing
the Notes and, until such time as the resale of the Conversion
Shares have been registered under the 1933 Act, the stock
certificates representing the Conversion Shares, except as set
forth below, shall bear any legend as required by the “blue
sky” laws of any state and a restrictive legend in
substantially the following form (and a stop-transfer order may be
placed against transfer of such certificates and
instruments):

NEITHER THIS NOTE NOR THE SECURITIES
INTO WHICH THIS NOTE IS CONVERTIBLE HAVE BEEN REGISTERED UNDER THE
SECURITIES ACT OF 1933, AS AMENDED, OR APPLICABLE STATE SECURITIES
LAWS. THE SECURITIES MAY NOT BE OFFERED FOR SALE, SOLD, TRANSFERRED
OR ASSIGNED IN THE ABSENCE OF (A) AN EFFECTIVE REGISTRATION
STATEMENT FOR THE SECURITIES UNDER THE SECURITIES ACT OF 1933, AS
AMENDED, AND APPLICABLE STATE SECURITIES LAWS, OR (B) THE
TRANSFEROR PROVIDES THE COMPANY WITH REASONABLE ASSURANCES AND
CUSTOMARY REPRESENTATIONS THAT REGISTRATION IS NOT REQUIRED UNDER
SAID ACT AND APPLICABLE STATE SECURITIES LAWS.

3

The legend set forth above shall be removed and
the Company shall issue a certificate or instrument without such
legend to the holder of the Securities upon which it is stamped,
if, unless otherwise required by state securities laws,
(i) such Securities are registered for resale under the 1933
Act; provided, that each Buyer has complied with or covenants and
agrees that it will comply with the prospectus delivery
requirements of the 1933 Act as applicable to it in connection with
sales of such Securities pursuant to a registration statement or
(ii) in connection with a sale, assignment or other transfer,
such holder provides the Company with reasonable assurances and
customary representations to the effect that such legend is not
required under applicable requirements of the 1933 Act.

(i) Validity; Enforcement .
This Agreement and Pledge and Security Agreement have been duly and
validly authorized, executed and delivered on behalf of such Buyer
and shall constitute the legal, valid and binding obligations of
such Buyer, enforceable against such Buyer in accordance with their
respective terms, except as such enforceability may be limited by
general principles of equity or to applicable bankruptcy,
insolvency, reorganization, moratorium, liquidation and other
similar laws relating to, or affecting generally, the enforcement
of applicable creditors’ rights and remedies.

(j) No Conflicts . The
execution, delivery and performance by such Buyer of this Agreement
and the Pledge and Security Agreement to which such Buyer is a
party and the consummation by such Buyer of the transactions
contemplated hereby and thereby will not (i) result in a
violation of the organizational documents of such Buyer or
(ii) conflict with, or constitute a default (or an event which
with notice or lapse of time or both would become a default) under,
or give to others any rights of termination, amendment,
acceleration or cancellation of, any agreement, indenture or
instrument to which such Buyer is a party, or (iii) result in
a violation of any law, rule, regulation, order, judgment or decree
(including federal and state securities laws) applicable to such
Buyer, except in the case of clauses (ii) and (iii), for such
conflicts, defaults, rights or violations which would not,
individually or in the aggregate, reasonably be expected to have a
material adverse effect on the ability of such Buyer to perform its
obligations hereunder.

(k) Residency . Such Buyer is
a resident of that jurisdiction specified in its address on the
Schedule of Buyers.

3. REPRESENTATIONS AND
WARRANTIES OF THE COMPANY .

The Company represents and warrants
to each of the Buyers that:

(a) Organization and
Qualification . Each of the Company and each Subsidiary is a
corporation duly organized, validly existing and in good standing
under the laws of the jurisdiction in which it is organized and has
all requisite corporate power and authority to carry on its
business as now conducted. Each of the Company and each Subsidiary
is duly qualified to transact business and is in good standing in
each jurisdiction in which the failure so to qualify would have a
Material Adverse Effect. As used in this Agreement, “
Material Adverse Effect ” means any material adverse
effect on the business, properties, assets, operations, results of
operations, or financial condition of the Company and its
subsidiaries taken as a whole.

4

(b) Authorization; Enforcement;
Validity . The Company has the requisite power and authority to
enter into and perform its obligations under this Agreement, the
Notes, the Pledge and Security Agreements and the Guaranty
Agreement (collectively, the “ Transaction Documents
”) and to issue the Securities in accordance with the terms
hereof and thereof. The execution and delivery of the Transaction
Documents by the Company and the consummation by the Company of the
transactions contemplated hereby and thereby, including, without
limitation, the issuance of the Notes, the reservation for issuance
and the issuance of the Conversion Shares issuable upon conversion
of the Notes, and the pledging of the Antigenics MA Shares have
been duly authorized by the Company’s Board of Directors and
no further corporate action is required by the Company. This
Agreement and the other Transaction Documents have been duly
executed and delivered by the Company, and constitute the legal,
valid and binding obligations of the Company, enforceable against
the Company in accordance with their respective terms, except as
such enforceability may be limited by general principles of equity
or applicable bankruptcy, insolvency, reorganization, moratorium,
liquidation or similar laws relating to, or affecting generally,
the enforcement of applicable creditors’ rights and
remedies.

(c) Issuance of Securities .
The issuance of the Notes and the pledging of the Antigenics MA
Shares have been duly authorized by the Company. As of the Closing,
a number of shares of Company Common Stock shall have been duly
authorized and reserved for issuance which equals the sum of 100%
of the maximum number of shares of Company Common Stock issuable
upon conversion of the Notes on the Closing Date. Upon issuance or
conversion in accordance with the Notes, the Conversion Shares will
be validly issued, fully paid and nonassessable. Assuming the
accuracy of the Buyers’ representations and warranties
contained herein, the offer and issuance by the Company of the
Notes will be exempt from registration under the 1933
Act.

(d) No Conflicts . The
execution, delivery and performance of the Transaction Documents by
the Company and the consummation by the Company of the transactions
contemplated hereby and thereby (including, without limitation, the
issuance of the Notes, the pledging of the Antigenics MA Shares and
reservation for issuance and issuance of the Conversion Shares)
will not (i) result in a violation of the Certificate of
Incorporation (as defined in Section 3(o)) or Bylaws (as
defined in Section 3(o)) or (ii) conflict with, or
constitute a default (or an event which with notice or lapse of
time or both would become a default) under, or give to others any
rights of termination, amendment, acceleration or cancellation of,
any agreement, indenture or instrument to which the Company is a
party, or (iii) result in a violation of any law, rule,
regulation, order, judgment or decree (including federal and state
securities laws and regulations and the rules and regulations of
the Nasdaq Stock Market (the “ Principal Market
”)) applicable to the Company or by which any property or
asset of the Company is bound or affected, except in the case of
clauses (ii) and (iii), for such conflicts, defaults, rights
or violations which would have a Material Adverse
Effect.

(e) Consents . Except as
disclosed in Schedule 3(e) and except for those failure of which to
obtain or make would not have a Material Adverse Effect, and any
filings required pursuant to Section 4(b) hereof, the Company
is not required to obtain any consent, authorization

5

or order of, or make any filing or registration
with, any court, governmental agency or any regulatory or any other
Person in order for it to execute, deliver or perform any of its
obligations under or contemplated by the Transaction Documents, in
each case in accordance with the terms hereof or thereof. All
consents, authorizations, orders, filings and registrations which
the Company is required to obtain pursuant to the preceding
sentence have been obtained or effected on or prior to the Closing
Date.

(f) Acknowledgment Regarding
Buyer’s Purchase of Securities . The Company acknowledges
that no Buyer is acting as a financial advisor or fiduciary of the
Company (or in any similar capacity) with respect to the
Transaction Documents and the transactions contemplated hereby and
thereby, and any advice given by a Buyer or any of its
representatives or agents in connection with the Transaction
Documents and the transactions contemplated hereby and thereby is
merely incidental to such Buyer’s purchase of the Securities.
The Company further represents to each Buyer that the
Company’s decision to enter into the Transaction Documents
has been based solely on the independent evaluation by the Company
and its representatives.

(g) No General Solicitation;
Placement Agent’s Fees . Neither the Company, nor any
Person acting on its behalf, has engaged in any form of general
solicitation or general advertising (within the meaning of
Regulation D) in connection with the offer or sale of the Notes. No
broker has acted on behalf of the Company in connection with this
Agreement, and there are no brokerage commissions, finders’
fees or similar fees or commissions payable in connection therewith
based on any agreement, arrangement or understanding with the
Company or any action taken by the Company.

(h) No Integrated Offering .
Neither the Company, nor any Person acting on its behalf, has,
directly or indirectly, made any offers or sales of any security or
solicited any offers to buy any security, under circumstances that
would require registration of the Notes to the Buyers under the
1933 Act or cause this offering of the Notes to be integrated with
prior offerings by the Company for purposes of the 1933
Act.

(i) SEC Documents; Financial
Statements . Since December 31, 2005, the Company has
filed all reports, schedules, forms, statements and other documents
required to be filed by it with the SEC pursuant to the reporting
requirements of the Securities Exchange Act of 1934 (the “
1934 Act ”) (all of the foregoing and all exhibits
included therein and financial statements, notes and schedules
thereto and documents incorporated by reference therein being
hereinafter referred to as the “ SEC Documents
”). As of their respective dates, the SEC Documents complied
in all material respects with the requirements of the 1934 Act and
the rules and regulations of the SEC promulgated thereunder and
applicable thereto, and none of the SEC Documents, at the time they
were filed with the SEC, contained any untrue statement of a
material fact or omitted to state a material fact required to be
stated therein or necessary in order to make the statements
therein, in the light of the circumstances under which they were
made, not misleading. As of their respective dates, the financial
statements of the Company included in the SEC Documents complied as
to form in all material respects with applicable accounting
requirements and the published rules and regulations of the SEC
with respect thereto. Such financial statements have been prepared
in accordance with generally accepted accounting principles,
consistently applied, during the periods involved (except
(i) as may be otherwise indicated in such financial

6

statements or the notes thereto, or (ii) in
the case of unaudited interim statements, to the extent they may
exclude footnotes or may be condensed or summary statements) and
fairly present in all material respects the financial position of
the Company as of the dates thereof and the results of its
operations and cash flows for the periods then ended (subject, in
the case of unaudited statements, to normal year-end audit
adjustments).

(j) Absence of Certain
Changes . Except as disclosed in the SEC Documents since
December 31, 2005, there has been no material adverse change
and no material adverse development in the business, properties,
assets, operations, results of operations, or financial condition
of the Company or any Subsidiary.

(k) Regulatory Permits . Each
of the Company and each Subsidiary possesses all certificates,
authorizations and permits issued by the appropriate federal, state
or foreign regulatory authorities necessary to conduct its
business, except where the failure to possess such certificates,
authorizations or permits would not have a Material Adverse Effect.
Each of the Company and each Subsidiary is not in violation of any
judgment, decree or order or any statute, ordinance, rule or
regulation applicable to it, except for violations which would not
have a Material Adverse Effect.

(l) Foreign Corrupt Practices
. Neither the Company or Subsidiary, nor any director, officer,
agent, employee or other Person acting on behalf of the Company or
Subsidiary has, in the course of its actions for, or on behalf of,
the Company or the Subsidiary (i) used any corporate funds for
any unlawful contribution, gift, entertainment or other unlawful
expenses relating to political activity; (ii) made any direct
or indirect unlawful payment to any foreign or domestic government
official or employee from corporate funds; (iii) violated or
is in violation of any provision of the U.S. Foreign Corrupt
Practices Act of 1977, as amended; or (iv) made any unlawful
bribe, rebate, payoff, influence payment, kickback or other
unlawful payment to any foreign or domestic government official or
employee.

(m) Sarbanes-Oxley Act . The
Company is in compliance with any and all applicable requirements
of the Sarbanes-Oxley Act of 2002 that are effective as of the date
hereof, and any and all applicable rules and regulations
promulgated by the SEC thereunder that are effective as of the date
hereof, except where such noncompliance would not have a Material
Adverse Effect.

(n) Transactions With
Affiliates . Except as disclosed in Schedule 3(n) and except as
set forth in the SEC Documents, and other than the grant of stock
options and restricted and non-restricted stock grants disclosed
that are required to be publicly disclosed, none of the officers,
directors or employees of the Company is presently a party to any
transaction with the Company (other than for ordinary course
services as employees, officers or directors) required to be
disclosed pursuant to Regulation S-K Item 404, including any
contract, agreement or other arrangement providing for the
furnishing of services to or by, providing for rental of real or
personal property to or from, or otherwise requiring payments to or
from any such officer, director or employee or, to the knowledge of
the Company, any corporation, partnership, trust or other entity in
which any such officer, director, or employee has a substantial
interest or is an officer, director, trustee or partner, which such
transaction would be required to be disclosed.

7

(o) Equity Capitalization .
The capitalization of the Company as of September 30, 2006 is
as set forth on Schedule 3(o) hereto. All the outstanding shares of
Company capital stock have been validly issued and are fully paid
and nonassessable. Except as disclosed in Schedule 3(o):
(i) none of the Company’s share capital is subject to
preemptive rights that would be triggered upon issuance of the
Securities; (ii) except as disclosed in the Form 10-K of the
Company filed in March 2006 with the SEC or subsequently filed
Forms 10-Q or Forms 8-K, there are no outstanding options,
warrants, scrip, rights to subscribe for, calls or other rights
convertible into, or exercisable or exchangeable for, any share
capital of the Company by which the Company is bound to issue
additional share capital of the Company, and (iii) there are
no outstanding debt securities, notes, credit agreements, credit
facilities or other agreements, documents or instruments evidencing
Indebtedness (as defined in Section 3(p)) by which the Company
is bound. The Company has made available to each Buyer true,
correct and complete copies of the Company’s Certificate of
Incorporation, as amended and as in effect on the date hereof (the
“ Certificate of Incorporation ”), and the
Company’s Bylaws, as amended and as in effect on the date
hereof (the “ Bylaws ”).

(p) Indebtedness and Other
Contracts . Except as disclosed in Schedule 3(p) or in the SEC
Documents, each of the Company and each Subsidiary (i) has no
outstanding Indebtedness (as defined below) other than Permitted
Indebtedness (as defined in the Note), including Indebtedness
secured by Permitted Liens, and (ii) is not in violation of
any term of or in default under any contract, agreement or
instrument relating to any Indebtedness, except where such
violations and defaults would not result in a Material Adverse
Effect. For purposes of this Agreement: (x) “
Indebtedness ” of any Person means, without
duplication (A) all indebtedness for borrowed money,
(B) all obligations issued, undertaken or assumed as the
deferred purchase price of property or services (including, without
limitation, “capital leases” in accordance with
generally accepted accounting principals) (other than trade
payables entered into in the ordinary course of business),
(C) all reimbursement or payment obligations with respect to
letters of credit, and surety bonds, (D) all obligations
evidenced by notes, bonds, or debentures, (E) all indebtedness
created or arising under any conditional sale or other title
retention agreement, or incurred as financing, in either case with
respect to any property or assets acquired with the proceeds of
such indebtedness (even though the rights and remedies of the
seller or bank under such agreement in the event of default are
limited to repossession or sale of such property), (F) all
monetary obligations under any leasing or similar arrangement
which, in connection with generally accepted accounting principles,
consistently applied for the periods covered thereby, is classified
as a capital lease, (G) all indebtedness referred to in
clauses (A) through (F) above secured by any mortgage,
lien, pledge, charge, or security interest upon or in any property
or assets (including accounts and contract rights) owned by any
Person, even though the Person which owns such assets or property
has not assumed or become liable for the payment of such
indebtedness, and (H) all Contingent Obligations in respect of
indebtedness or obligations of others of the kinds referred to in
clauses (A) through (G) above; (y) “
Contingent Obligation ” means, as to any Person, any
direct or indirect liability, contingent or otherwise, of that
Person with respect to any indebtedness, lease, or other monetary
obligation of another Person if the primary purpose or intent of
the Person incurring such liability is to provide assurance to the
obligee of such liability that such liability will be paid or
discharged; and (z) “ Person ” means an
individual, a limited liability company, a partnership, a joint
venture, a corporation, a trust, an unincorporated organization and
a government or any department or agency thereof.

8

(q) Litigation . Except as
set forth in Schedule 3(q) or in the SEC Reports, there is no
action, suit, proceeding, inquiry or investigation before any
court, public board, government agency, self-regulatory
organization or body pending other than those which would not have
a Material Adverse Effect. Each of the Company and Antigenics MA,
has, in all material respects, complied with all laws, regulations
and orders applicable to its business, including Pharmaceutical
Laws (as defined below), and has all material permits and licenses
required thereby. For purposes of this Agreement, “
Pharmaceutical Law ” shall mean any federal, state,
local or foreign law, statute, rule or regulation relating to the
development, commercialization and sale of pharmaceutical and
biotechnology products and devices, including all applicable
regulations of the U.S. Food and Drug Administration and comparable
applicable foreign regulatory authorities.

(r) Insurance . Each of the
Company and each Subsidiary is insured by insurers of recognized
financial responsibility against such losses and risks and in such
amounts as are prudent and customary in the businesses in which
either the Company or Subsidiary is engaged.

(s) Employee Relations . The
Company is not a party to any collective bargaining agreement. The
Company is in comp

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